How will the world cope if the H5N1 bird flu virus mutates and a humanised strain materialises?

Many scientists believe this is an almost certain event. It is not if, it is when. Are we prepared? Who will be saved? Is this a 21st Century plague?

In a 90-minute docu-drama, BBC Two's Horizon programme paints an apocalyptic vision of the future, portraying the horror of a modern-day influenza pandemic in terrifying detail.

The film shows the latest developments in flu research and reveals the US government is asking its citizens to stockpile medicines and food in preparation for an outbreak.

The British government's Influenza Pandemic Contingency Plan paints a scenario of up to three-quarters of a million British deaths. The US is predicting close to two million fatalities.

Horizon tells the story of how a pandemic could unfold.

Cities silent

It predicts hospitals and emergency services will be overwhelmed, with insufficient capacity to dispose of the bodies of those who die.

The world's once busy cities will lie eerily silent. A quarter of the workforce will be absent. Public transport will all but disappear, schools will close and public gatherings will be banned.

This vision is not science fiction. It is based on the latest scientific research, which forms the basis for government policy across the globe. Contingency plans for pandemic flu predict millions of deaths, economic meltdown and society in chaos.

The public perception is that only the elderly and weakest members of society die from influenza. But a devastating flu pandemic may target people in the prime of life - the 20 to 40 age range.

We can only vaccinate about 600 million people from every six months of global production

Prof Peter Dunnill

If a virus does arrive, it will take five to six months to produce a working vaccine.

Professor Peter Dunnill, of University College London, says: "The amount of pandemic vaccine that we produce globally is quite small.

"We can only vaccinate about 600 million people from every six months of global production. To put that in perspective, that's about 9% of the global population."

At present, government plans talk of the need to manage public expectation and list key workers who will qualify for early vaccination.

The final order of priority has yet to be confirmed - deciding who goes where on the list will not be a task for the faint-hearted.

It has been suggested that the elderly and very young should be denied vaccine until after all other groups have been vaccinated because they have either had a full life, or have not been invested in.

Difficult decisions

Prof Dunnill says: "You have to give vaccine to frontline medical staff, because they're going to be face-to-face with the people who are plainly infected.

"But if you get to a situation where you have to make decisions within the public, it's much more difficult. Scientists have suggested that two-year-olds may not be people that have high priority because, as yet, there hasn't been a sizeable investment.

"Now those are brutal decisions but people are having to think in those sorts of terms."

[In 1917] you had a virus that didn't seem to spread at all but within a year it exploded and killed 50 million people. So there's a warning there. We cannot ignore a virus that has done that in the past

Prof John Oxford

Prof Dunnill also questions the effectiveness of anti-viral drug Tamiflu.

"At the present time, of course, Tamiflu is used for ordinary flu and we know that even there it has to be used quickly to be effective.

"What we don't know is what would happen with H5N1. The present evidence coming from Vietnam and Indonesia and countries like that is not particularly good but of course, a lot of the patients are travelling considerable distances before they're treated.

"We still don't have evidence of any real detail on the effect when Tamiflu is given very quickly."

The H5N1 virus bears a striking resemblance to the influenza pandemic that struck after the First World War, killing 50 million people.

The H5N1 virus has an extra piece of genetic code that some scientists say makes it much more deadly than 1918 virus.

Pandemic will be shown on BBC Two on 7 November at 2100 GMT as part of the Horizon series.